The effect of heat treatment on microstructure and creep property of Ti-45 and 47Al-2Nb-2Mn+0.8vol.%TiB2 alloys (noted as 45XD and 47XD) has been studied. Oil quench can produce fully lamellar structure with fine lamellar colonies and lamellae for both alloys. Different aging treatments were applied to oil quenched samples in order to stabilize the fully lamellar structure. Hardness of both alloys decreases after the aging treatments. Creep tests of unaged and aged samples were conducted at 760oC and 207 MPa. The creep test results show that the aged samples have lower creep strain rates in primary creep stage and longer rupture lives than the unaged samples for both alloys. Microstructure analysis of the samples after aging and long-term thermal exposure during creep indicates that selected aging treatments stabilized the lamellar structure. During aging several modes of a2 to g phase transformation took place. Different phase transformation modes have different effects on properties. The effect of heat treatment on creep property will be discussed on the basis of microstructural evolution during the heat treatment and the creep.